1. Field of the Invention
The present invention relates to a process for producing silicon nitride or silicon carbide or both (collectively referred to as silicon ceramics hereinafter) in the form of a fine powder.
2. Discussion of the Background
Silicon ceramics are used in various fields of application because of their low coefficient of thermal expansion, high strength at high temperatures, and superior heat shock resistance. When it is desired to prepare silicon ceramics which exhibit increased catalytic activity, or when it is desired to effectively sinter silicon ceramics at low temperatures, the silicon ceramic should be powdered to a particle diameter less than 1000 .ANG..
Conventionally, silicon nitride powder is produced by reacting metallic silicon directly with nitrogen as described in reaction scheme (1) as follows: EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 ( 1)
Silicon nitride is also produced by reacting silicon tetrachloride with ammonia, as shown in reaction scheme (2) as follows: EQU 3SiCl.sub.4 +3NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12HCl (2)
Still further, silicon carbide powder can be produced by reacting silicon dioxide with carbon, as shown in reaction scheme (3) as follows: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (3)
Silicon carbide can also be produced by reacting silicon tetrachloride with methane as shown in reaction scheme (4) as follows: EQU SiCl.sub.4 +CH.sub.4 .fwdarw.SiC+4HCl (4)
However, all of the above-described processes have problems. The reaction of scheme (1) above requires metallic silicon of high purity in finely powdered form, and produces a silicon nitride powder product which is constituted of a broad particle size distribution. This particular reaction does not yield a product powder finer than 1000 .ANG. except with great difficulty. In addition, the scheme inevitably results in the incorporation of impurities in the ceramic product, because of the limited availability of pure metallic silicon in finely powdered form. The third reaction scheme requires high temperatures in excess of 2000.degree. C., which means that large amounts of heat energy are required. The process also yields a coarse silicon carbide powder product which must subsequently be pulverized. The resulting powder has a broad particle size distribution and has very few particles of a diameter smaller than 1000 .ANG.. Further, the pulverization step which is required results in impurity incorporation in the ceramic product and results in increased production costs. Finally, the second and fourth processes require expensive silicon tetrachloride as a starting material and give off harmful hydrogen chloride gas that has to be properly disposed of. All of these factors increase the price of silicon ceramics. A need therefore continues to exist for a method of simply and economically producing finely powdered silicon ceramics.